Check fringes laserdiode

On: Tue, Apr 16, 02 03:27:53 PM

Ton wrote:

My way to shoot holograms is now: close behind the outlet
of the lasermodule I 'steal' about 10% of the laserbeam
with a glasplate. This 10%-beam I use in a Michelsen
interferometer and project the fringes on a white plate.
The interferometer is carded off from the holographic
setup. The 90%-beam is used to make the holograms.
In this way you can perfectly time your exposures, since
you can see when the fringes stand still and have maximum
contrast. As I have observed a maximum exposure of 5
minutes should be no big deal anymore.

RESPONSES

Frank - Wed, Apr 17, 02 02:19:21 PM

This is a very interesting idea -- similar to stealing part
of the beam for an "active" fringe-locker, but passive in
that it is visible to the eye (as an indicator of fringe
stability periods on the white card) without any electronic
correction/compensation.
Not sure how you have this set up, but having the beam paths
of the interferometer the length of the entire table would be
valuable as well -- and especially useful right now (spring)
in the N. Hemisphere -- as temps. and humidity begin to rise,
and amateur/hobbyist tables begin to "bend and twist" as a
result (especially wood-based tables).
Overall, a great little way to catch those "moments" when
everything comes together . . . and it's time to release that
shutter.
Frank
63.65.0.107

Sergio Oliveira - Fri, Apr 26, 02 07:27:53 PM

I use a different aproach, instead the Michelson
interferometer, a simple glass etalon will have the same
effect, used with the diverging bean withouth lens, the
output is the same 10%, the advanced feature is the
superior interferometer stability due the etalon itself,
any fringe movement and noise can be monitored.200.158.172.58

As your glassplate, the etalon steal a 10% laser beam at an
angle that refects the beam back. This is a plane parallel
glass plate 2mm thick that faces act as a Fabry-Perot
interferometer, at one position you will see the Newton
rings projected on the wall by the interferometric faces,
any wavelength sweep and noise could be monitorated pretty
well, all those external "movements" are filtered out, the
laser woth etalon itself became as one integral piece, with
this, I have the possibility to monitorate the effect of
air currents on wavelength stability of my 30mW laser!Now I
know the parameters for a modereate exposure time, >10min,
and the laser settle time.
200.158.172.102

Tom B. - Sun, Apr 14, 02 03:47:29 AM

Sergio - thanks. - I'd seen this effect before
but hadn't thought to look at how useful
it would be for stability checks. Some
observations:
Microscope slides (0.8 mm) and cover slips (0.13 mm)
work pretty well. The circular bullseye pattern is
reflected back toward the laser. A white card with a hole
for the laser makes a nice projection surface. The
fringe spacing is proportional to glass thickness
(thinner = wider spacing) and distance from the
laser. If the glass faces are not perfectly parallel,
the bullseye is not centered on the laser source, and
rotates around the source when you rotate the glass.
Interestingly, tilting the glass does not affect the
position or appearance of the pattern much.
I'm just getting an LNCQO5 diode going, and noticed that
above 30 mW output, fringes started to disappear with
the 0.8 mm slide but were still visible up to 50 mW
with the 0.13 mm cover slip, so as expected the thickness
of the glass determines the coherence length that it is
sensitive to. I tried a 5 mm piece of glass, and saw the
same thing as with 0.8 mm. I'll have to set up an
interferometer to see how coherence behaves over longer path
differences.
I'm hoping that the glass plate will be a useful diagnostic
substitute for an interferometer.
24.67.253.203

Sergio - Tue, May 28, 02 10:44:15 PM

This is an interesting instrument, precisely the Fabry
Perot interferometer..all the facts that you related are
well studied in this instrument, according to one formula,
the etalon thickness determine the laser longitudinal
linewidth, you could determine not only the laser
longitudinal mode quality but all noise effects that will
affect the diode sensitivity.
The experiment that you related is very well delined, I
think with the etalon "physics" you could manipulate
precisely this instrument to precisely stabilish the
coherence for diode holography.
The tilted etalon is common used in dye laser selectivity,
if you are interested in the etalon mathematics I could
indicate one book in laser that is useful.
serg1@softhome.net200.158.172.81

Colin Kaminski - Wed, May 29, 02 11:54:45 AM

I have noticed the line width changes just before the diode
goes in to an unstable zone. (mode hop) Looking through my
books I do not have any Optics books with the math to sort
out linewidth from a etalon. Can you post the title of the
book you would recommend?64.167.149.193

Tom B. - Wed, May 29, 02 03:36:59 PM

Do you mean spectral linewidth? How are you measuring
this? Or are you maybe referring to fringe
sharpness/contrast or fringe spacing?
24.67.253.203

Colin - Thu, May 30, 02 11:15:13 AM

I think I mean spectral line width. What I am measuring is
coherence length. Am I mistaken to think they are really
the same thing? Fringe spacing is related to frequency.
With my temperature controller I can adjust the fringe
spacing by changing temperature.64.170.192.157

Tom B. - Thu, May 30, 02 10:04:22 PM

OK, yes, linewidth and coherence length are reciprocally
related. I guess I was confused since the thread was
discussing reflected glass plate fringes and it wasn't
clear how you would get linewidth or coherence length
beyond the thickness of the glass from observing these.
Unless you just meant that the fringes start to fade before
mode hopping sets in.24.67.253.203

Colin - Thu, May 30, 02 11:23:24 PM

When I read Sergio's post I think he implies you can either
measure or adjust the line width with a etalon. I would be
very interested in figuring this out.64.170.192.157

Sergio - Thu, Jun 06, 02 06:28:49 PM

Colin, for this external etalon configuration you could
only monitorate (precisely) the linewidth and mode
stability in respect to quantity and quality, the fringe
contrast can display the general laser noise too, linewidth
is "more" related to mode numbers, practically for
holography only one mode is useful for good coherence
length, but you could find two or tree in HeNe laser
systems.
I think that well defined circular fringes, 0,1,0,1,0..
will define the longitudinal single mode, the transversal
is always TEM00, for a good beam quality,in HeNe lasers the
fringes are not equaly spaced in multimode lasers.
Fringes fades would be related to current noise, I suppose,
anyway the etalon will display this noise at laser diode
thermal building up. The book that is very dictating in
this respect is: "Solid State Laser Engineering" by Walter
Koechner, Springer Verlag ISBN: 0-387-53756-2
See specially chapter 5.2.
200.158.172.230